1. Field of the Invention
This invention relates to an image signal processing apparatus for use in a system such as facsimile and in particular to an image signal processing apparatus for converting an analog image signal obtained by reading an original into a binary image signal for further processing such as transmission to a remote place.
2. Description of the Prior Art
An image signal processing apparatus for converting an analog image signal into a binary image signal has been well known and the structure of such a typical prior art apparatus is schematically illustrated in FIG. 1. As shown, the prior art apparatus comprises an amplifier 1, to the positive input terminal of which is supplied an analog image signal as obtained by reading an original, a capacitor 2 for holding the peak value of the analog image signal amplified by the amplifier 1, a voltage divider comprised of resistors 3 and 4, and a comparator 5 which compares the reference voltage from the voltage divider with the analog image signal and supplies a binary image signal "1" or "0" when the level of the analog image signal exceeds that of the reference voltage.
The output of the voltage divider, or the reference voltage, is determined to the value which is a fraction of the analog image signal level according to the resistance values of the resistors 3 and 4. Thus, the reference voltage varies in proportion to the peak value of the analog image signal. It should, however, be noted that the contrast between the background and image areas varies from original to original. Therefore, if the reference voltage is fixed at a constant proportionate value in relation to the level of the analog image signal as in the prior art apparatus, it often happens that the converted binary image signal is replete with noise or misses some information.
In other words, an original may have a small contrast between the white and black levels in the analog image signal as shown in FIG. 2a; whereas, another original may have a large contrast as shown in FIG. 2b. Suppose that the peak level is the same for the analog image signals shown in FIGS. 2a and 2b. Then, the reference, or threshold, level is the same for the signals shown in FIGS. 2a and 2b as indicated by the dashed line. Under the circumstances, in the case of the analog image signal shown in FIG. 2a, some information will be lost when converted into the binary image signal. If the reference level is raised closer to the peak level, it is true that more information may be picked up, however, in the case of the original having a larger contrast between the white and black levels as shown in FIG. 2b, black level information becomes predominant in the converted binary image signal, resulting in the production of "crushing" or widening of the lines of reproduced characters or diagrams.
In accordance with one approach which has been proposed to cope with the above-mentioned problem, one or both of the resistors 3 and 4, forming together a voltage divider, should be made variable. With such a structure, the proportion of the reference level to the level of the analog image signal may be adjusted appropriately in theory. However, it is quite impractical because such adjustments must be manually carried out for each original and the resistors are mounted on the printed circuit board which is usually located in the interior of the apparatus, providing only a limited accessibility. As a solution to this impracticality, the variable resistors 3 and 4 may be mounted on the control panel for easy handling by the operator. This again brings about another problem because extended leads are required for interconnections with the capacitor 2 and the comparator 5, which could make the reference voltage fluctuate when influenced by the increased stray capacitances and increase the possibility of introduction of noises, bringing about deterioration of the quality of a reproduced image.
There has been proposed another approach which adjusts the reference voltage from the control panel with ease in accordance with the condition of an original without the use of the extended interconnections. One example of this approach is shown in FIG. 3 wherein four resistors 7.sub.1 through 7.sub.4 are connected in series to form a voltage divider and each junction between adjacent resistors is connected to the comparator 5 through a relay unit 9, including normally open relays, 9.sub.1 and 9.sub.3 and a normally closed relay 9.sub.2. A switch unit 8 including two switches 8.sub.1 and 8.sub.2 is connected to the relay unit 9. The switch unit 8 is mounted on the control panel and the individual swithches 8.sub.1 and 8.sub.2 may be operated by the operator at the control panel. When all of the switches 8.sub.1 and 8.sub.2 are off, only the normally closed relay 9.sub.2 is on, and, therefore, the reference voltage appearing at the junction between the resistors 7.sub.2 and 7.sub.3 is supplied to the comparator 5. When the switch 8.sub.1 is closed, the normally closed relay 9.sub.2 is turned off and the normally open relay 9.sub.1 is turned on so that the higher reference voltage appearing at the junction between the resistors 7.sub.1 and 7.sub.2 is supplied to the comparator 5. On the other hand, when the switch 8.sub.2 is closed, the normally closed relay 9.sub.2 is turned off and the normally open relay 9.sub.3 is turned on, whereby the lower reference voltage at the junction between the resistors 7.sub.3 and 7.sub.4 is supplied to the comparator 5.
Accordingly, it is now possible to selectively supply an appropriate reference voltage to the comparator 5 by operating the switch unit 8 in accordance with the contrast between the background area and the image area of an original and the kind of an original. For example, if an original has a high-density information such as a newspaper, the switch 8.sub.2 must be turned on since there is a chance of causing "crushing" phenomenon in a reproduced image. On the other hand, if an original has a low-density information such as a diagram, the switch 8.sub.1 must be turned on. For an original with an average-density information, both of the switches 8.sub.1 and 8.sub.2 must be turned off. If there is discoloration in the background area, lowering the contrast between the background and image areas, the switch 8.sub.1 must be turned on or closed. With such a structure, the reference voltage may be easily adjusted from outside of an image sensing device such as in a facsimile system, thereby allowing a reproduced image of high quality.
It is to be noted, however, that the image signal processing device of FIG. 3 is not free from disadvantages. That is, if it is desired to carry out fine adjustments in response to the contrast between the background and image areas and/or to an increased number of types of originals, an unacceptably increased number of switches and control lines, connecting the switches to the relays, must be provided. This tends to make the apparatus bulky and expensive. Moreover, the increased number of components and wiring contributes to the occurrence of malfunctioning.